One type of conventional vehicle brake pressure control device includes a static pressure dual brake circuit. One of the brake circuits is formed with a first wheel brake that applies a brake force proportional to a fluid pressure to a first wheel, a second wheel brake that applies a brake force proportional to a fluid pressure to a second wheel, a reservoir which stores brake fluid under atmospheric pressure, and a master cylinder. The master cylinder is connected to the reservoir containing brake fluid, generates fluid pressure in proportion to the brake pedal operation force, and applies the fluid pressure to the wheel brakes.
This conventional vehicle brake pressure control device further includes a first normally-open type electromagnetic valve disposed on a first fluid conduit connecting the master cylinder to the first wheel brake, a second normally-open type electromagnetic valve disposed on a second fluid conduit connecting the master cylinder to the second wheel brake, a first normally-closed type electromagnetic valve disposed on a third fluid conduit connecting the first wheel brake to the reservoir, a second normally-closed type electromagnetic valve disposed on a fourth fluid conduit connecting the second wheel brake to the reservoir, and a fluid pump which returns brake fluid to portions of the first and second fluid conduits connecting the master cylinder and each of the first and second normally-open type electromagnetic valves.
In this conventional vehicle brake pressure control device, at a point approximately when the first wheel would tend to lock (the "lock tendency point") because of the brake force generated by the brake operation of a vehicle driver, the first normally-open type electromagnetic valve is closed to interrupt the supply of brake fluid to the first wheel brake from the master cylinder, and the first normally-closed type electromagnetic valve is opened to discharge pressurized brake fluid in the first wheel brake to the reservoir. When the rotational speed of the first wheel is restored past the lock tendency point by virtue of the decrease of the fluid pressure in the first wheel brake, the first normally-open type electromagnetic valve is opened and the first normally-closed type electromagnetic valve is closed again, thus once again applying fluid pressure generated by the master cylinder to the first wheel brake.
The fluid pump used in the above-described brake pressure control device is typically a piston pump which cannot draw in brake fluid under atmospheric pressure. The piston pump includes a mechanism for attenuating the pulsation of the pump discharge to prevent the driver from feeling uncomfortable with the brake pedal operation because of the transmission of the pulsation of the pump discharge to the brake pedal.
Another conventional vehicle brake pressure control device includes first and second wheel brakes which are disposed on first and second wheels respectively, with either or both of the wheels being driving wheels, a third normally-open type electromagnetic valve disposed on a fluid conduit connecting the master cylinder and the first and second normally-open type electromagnetic valves, a fluid pump which is able to draw in brake fluid under atmospheric pressure, and a third normally-closed type electromagnetic valve disposed on a conduit connecting a suction port of the fluid pump to a reservoir. This conventional brake system also includes first and second normally closed electromagnetic valves similar to those described above in connection with the first described conventional brake pressure control device.
According to this second type of conventional vehicle brake pressure control device, when the first wheel tends to be spun because of the drive force supplied to the first wheel, the second normally-open type electromagnetic valve, the third normally-open type electromagnetic valve and the third normally-closed type electromagnetic valve are operated and the fluid pump is driven to supply pressurized brake fluid from the reservoir to the first wheel brake. When the spinning of the first wheel is stopped by virtue of the supply of pressurized brake fluid to the first wheel brake, the first normally-open type electromagnetic valve is operated to interrupt the flow of pressured brake fluid into the first wheel brake from the fluid pump and the first normally-closed type electromagnetic valve is operated to relieve the fluid pressure in the first wheel brake to the reservoir, thereby decreasing the fluid pressure in the first wheel brake.
This second type of conventional vehicle brake pressure control device further includes a relief valve which regulates the maximum value of the discharge fluid pressure of the fluid pump, thereby relieving pressure when the fluid pressure exceeds a certain predetermined value.
A further conventional vehicle brake pressure control device includes a pre-charging pump disposed on a fluid conduit connecting the suction port of the fluid pump to the reservoir to rapidly increase the fluid pressure in the first wheel brake at the early stage of the fluid pump operation. Alternatively, instead of a pre-charging pump, another vehicle brake pressure control device includes an accumulator for storing high fluid pressure and an electromagnetic valve which controls the flow of fluid into and out of the accumulator. This conventional type of device involving the use of an accumulator is further provided with a surveillance apparatus for observing the fluid pressure in the accumulator. The surveillance apparatus drives the fluid pump when the fluid pressure in the accumulator is less than a predetermined minimum value, and stops the driving operation of the fluid pump when the fluid pressure in the accumulator exceeds a predetermined maximum value.
The above-described conventional vehicle brake pressure control devices suffer from the disadvantage that they tend to be rather complicated in construction, thus resulting in a brake circuit that is undesirably high in cost.